1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved method and apparatus for facilitating indirect heat transfer between a refrigerant and a cooled fluid. In still another aspect, the invention relates to a system for liquefying natural gas that employs at least one vertical core-in-kettle heat exchanger to cool the natural gas.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state; the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Many LNG facilities are located in relatively remote areas near natural gas reserves. When a new LNG facility is built in such a remote location it is common for the major components of the LNG facility to be manufactured in a more populated area and subsequently shipped (usually by ocean-going vessel) to the site of the LNG facility for final assembly. In order to save costs, it is desirable for the bulk of the complex components of the LNG facility to be constructed prior to shipping, so that most of the construction at the site of the LNG facility involves relatively simple assembly of the pre-fabricated complex components. However, as the capacity and size of LNG facilities increases, certain complex components have become too large to construct off-site and then ship to the final destination. One such component is known as a “cold box.”
A cold box is simply an enclosure that houses a plurality of refrigeration components (e.g., heat exchangers, valves, and conduits) that operate at a similar low temperature. In a typical cold box, the refrigeration components are assembled in the enclosure and surrounded by a flowable insulation (e.g., particles of expanded perlite) to insulated the multiple refrigeration components. Cold boxes provide a much more efficient and cost effective means for insulating multiple refrigeration components, verses individually insulating each component.
As alluded to above, it is much less expensive to assemble all of the components of a cold box in a more populated area and then ship the entire assembled cold box to the remote LNG facility site for installation. However, as LNG facilities have continued to increase in capacity and size, the size of cold boxes has also increased. In fact, some cold boxes are now too large to ship on standard ocean-going vessels. The main reason for the increased size of the cold boxes is that the conventional horizontal core-in-kettle heat exchangers disposed inside the cold boxes have increased in size to account for the higher cooling demand of larger LNG facilities. Thus, newly-constructed high-capacity LNG facilities utilizing conventional horizontal core-in-kettle heat exchangers require the cold box to be assembled on-site because a pre-assembled cold box would be too large to ship on a standard ocean-going vessel.
In addition to the size/space problems posed by conventional horizontal core-in-kettle heat exchangers, a number of heat transfer inefficiencies can be associated with such horizontal core-in-kettle heat exchangers. For example, the minimal liquid refrigerant depth provided below the core of the exchanger can hamper the availability of liquid refrigerant to the core. Also, the vertical distance between the top of the core and the upper gaseous refrigerant outlet of the shell may be too small to provide adequate disengagement of the gaseous and liquid phases of the refrigerant. When adequate liquid/gas disengagement above the core is not achieved, a significant amount of liquid refrigerant entrained in the upwardly-flowing gaseous refrigerant can undesirably exit the upper gaseous refrigerant outlet of the shell.